Although overmoded waveguides are generally recognized as undesirable in microwave systems, their employment has become necessary because of the need to minimize the losses and/or to accommodate multi-frequency operation in many modern microwave systems. This need for overmoded waveguides presents a problem, however, because the resulting higher-order modes generated in an overmoded waveguide make it more difficult to achieve another increasingly significant objective of modern microwave systems, namely, narrower radiation patterns required by today's crowded microwave spectrum.
In addition to the problem mentioned above, the higher-order modes generated by overmoded waveguide give rise to a group delay problem. That is, certain of the higher-order modes are re-converted to the desired mode, but only after they have traveled through the overmoded waveguide at different velocities, thereby producing desired mode signals which are not in phase with each other. This problem becomes more serious as the length of the overmoded waveguide is increased.